Throughout most of the world, electrical power is supplied to consumers as either 110-120 volts or 220-240 volts AC. A power supply in a consumer device rectifies the supplied AC input to produce a DC voltage, which it then uses to operate a connected device. The device may include separate power supplies, one dedicated to the 110-120 volt range and the other dedicated to the 220-240 volt range. Alternatively, to avoid manufacture of separate power supplies for each of the voltage ranges, a power supply may include a rectifying circuit which operates in one mode for a 220-240 volt AC input and in a second, or "doubler" mode, for a 110-120 volt AC input, such that it produces a predetermined DC voltage, for example, a DC voltage of (.sqroot.2)*220-240 volts, in response to either range AC input. The rectifying circuit includes a switch which, as discussed below, configures the circuit for the appropriate mode of operation.
If the AC input is 240 volts, the switch is set to configure the rectifying circuit in a non-doubler mode. In this mode, the rectifiers in the circuit are connected as a full-wave rectifying bridge and two bulk, or filter, capacitors in series across the bridge output terminals. .The bridge circuit charges the two capacitors in series during the peak portions of each half-cycle of the AC input and thus produces an output voltage which is equal to (.sqroot.2)*AC input. If the capacitors are of equal capacitance, each one charges to a potential of one-half of the output voltage.
If the input is 120 volts AC, the switch is set to configure the rectifying circuit in the doubler mode. In this mode, only part of the rectifying bridge passes current, such that the bridge charges one capacitor during each of the positive half-cycles of the AC input and the other capacitor during the negative half-cycles. Each capacitor charges to a potential equal to (.sqroot.2)*AC input, and a potential of twice that value is obtained across the two capacitors. The rectifying circuit thus produces, for an AC input of 110-120 volts, a DC output of (.sqroot.2)*220-240 volts.
As stated above, a switch typically controls the configuration of the rectifying circuit. The switch may be operated manually, and thus, set to the correct state by the manufacturer before shipment. Alternatively, the switch may be controlled automatically, such that it switches essentially instantaneously to the doubler mode configuration or the non-doubler mode configuration, as appropriate, as described in U.S. Pat. No. 4,654,538 to Lethellier.
As soon as the rectifier circuit is connected to an AC power source, i.e., plugged into a power outlet, the circuit draws a relatively large current to charge the bulk capacitors. This current, which is referred to as an "inrush" current, can be substantially larger than the current associated with steady state operation of the power supply. Accordingly, the power supply components must be capable of withstanding this large current.
A "soft start" circuit may be added between the power supply and the AC power source, as discussed in U.S. Pat. No. 4,328,459 to McLeod, Jr. The soft start circuit essentially operates a switch to allow increasingly larger portions of the supplied AC input to pass from the AC power source to the power supply while the capacitors are charging. This enables the capacitors to charge gradually, and they thus draw less inrush current. The soft start circuitry, which is dedicated circuitry which must be added to the power supply, increases the cost of the associated consumer device. What is needed is a mechanism to control inrush current which can be incorporated directly into the power supply circuitry, and which utilizes components already included in the power supply.